1. Field of the Invention
The present invention generally relates to an apparatus and method for diagnosing the electrical condition of an elongated casing, such as a metal pipe. More specifically, the present invention relates to borehole casing diagnostic apparatuses and methods which are suitable for evaluating cathodic protection of casings and for providing local corrosion rate measurements for casings which are fitted into boreholes traversing earth formations.
2. Prior Art
When a metal pipe or casing is fitted into a borehole, the metal will be liable to corrode as the fluids present in the borehole are potentially corrosive. Because an earth formation may include several layers, large scale electrochemical cells can be set up between the parts of the casing contacting the different layers, so that in some regions, net current enters the casing from the formation while in other regions net current leaves the casing and flows into the formation. When net current in the form of metal ions leaves the casing, the casing will corrode gradually. It is known that the rate of such corrosion can be in the order of mm/year, which is proportional to an outgoing current on the order of microamps/cm.sup.2. Since this electrochemical corrosion results from the current leaving the casing, the corrosion can be prevented if the casing is maintained at a potential such that net current enters the casing over its entire length. For this purpose, cathodic protection is often used. Cathodic protection is well known in the art, and prevents the electrochemical corrosion of a metal casing fitted in a borehole traversing an earth formation by transforming the metal casing into the cathode of an electrochemical cell.
In order to determine whether cathodic protection is needed for a casing, and in evaluating and/or optimizing the cathodic protection applied to a particular casing, the potential profile of the casing along its longitudinal direction is measured. In such a case, typically, a downhole apparatus which is provided with a plurality of electrodes is lowered into the casing. The downhole apparatus is suspended by a long cable which is connected to a measurement unit located at the ground surface, and measurements such as potential difference measurements are taken by the measurement unit at the ground surface as the downhole apparatus traverses the length of the casing. With such an arrangement, since the information of a potential difference between any two of the electrodes in contact with the casing must be transmitted to the measurement unit at the ground surface through the long cable connecting the downhole apparatus to the measurement unit, the information tends to be degraded due to noise pick-up and crosstalk, among other reasons. In addition, the information is also adversely affected by thermal EMFs along the long cable and at the electrical connections on the surface and in the downhole tool. In order to eliminate the effects of the cable as much as possible, and in particular to eliminate the thermal EMF effects, it has been proposed in U.S. Ser. No. 925,035, filed Oct. 30, 1986, which is assigned to the assignee herein and hereby incorporated herein by reference, to incorporate a relay switch in the downhole apparatus for subtracting the effects of the cable as described. While the proposed technique significantly improves the quality of data collected and the potential difference determinations, it still necessitates the carrying out of measurements through the long cable since the measurement unit is located at the ground surface. As long as measurements are carried out at the ground surface, measurement times will be long due to the need to average or integrate data because of crosstalk and noise problems which occur when remote measurements are made through long cables.
As set forth above, in order to carry out potential difference measurements along the longitudinal axis of a casing, the downhole apparatus is typically provided with a plurality of longitudinally spaced electrodes. Accordingly, it is either assumed that there is no appreciable potential difference or current in the circumferential direction or such effects are simply neglected. While U.S. Pat. No. 3,207,981 issued to Marsh et al. on September 21, 1965 discloses a downhole apparatus including a plurality of electrodes which are disposed circumferentially, it only measures artificially induced circumferential potentials and neglects the potential difference or current components in the longitudinal direction. Thus, none of the prior art techniques sufficiently guarantee the reliability of the electrical property data taken since they only provide for the making of measurements either in the longitudinal direction alone or in the circumferential direction alone while the potential difference or current may have components both in the longitudinal and circumferential directions.